HR: 08:15h
AN: V11E-02    [Abstracts]
TI: Dehydration Behavior of Metapelites and Metabasites at Very low to low Grade Metamorphic Conditions
AU: * Massonne, H
EM: h-j.massonne@imi.uni-stuttgart.de
AF: Institut fuer Mineralogie und Kristallchemie, Universitaet Stuttgart, Azenbergstr. 18, Stuttgart, 70174, Germany
AU: Willner, A P
EM: arne.willner@rub.de
AF: Institut fuer Mineralogie und Kristallchemie, Universitaet Stuttgart, Azenbergstr. 18, Stuttgart, 70174, Germany
AB: Thermodynamic calculations have been undertaken in the system Na-Ca-K-Fe-Mg-Al-Si-Ti-H-O with the PERPLE_X software package (Connolly, 1990 and updates) for a better understanding of the dehydration behavior of metapelites and metabasites during prograde metamorphism. To obtain reasonable results for the temperature range 150-450° C at pressures up to 25 kbar, the subsequent solid solution models were introduced being compatible with the applied thermodynamic data set of Holland & Powell (1998 and updates): a three component model for Mg-Fe²⁺-Fe³⁺-pumpellyite, a two component model for Fe²⁺-Mg- stilpnomelane, a four component amphibole model (tremolite - Fe²⁺-tremolite - glaucophane - Mg- riebeckite), and a four component Na-pyroxene model (acmite - jadeite - diopside - hedenbergite). The water contents released by prograde metamorphism up to 450° C from MORB and psammopelitic compositions on top of oceanic crust, were obtained by calculating P-T pseudosections. Metabasite contains 6-7 wt% H₂O bound to minerals at 150° C depending on the oxidation state. Along geotherms lower than 7° C/km typical for young subduction zones, no water is released up to 400° C. However, reduction of the rock causes release of small amounts of water. Metapsammopelitic rocks also store about 6 wt% H₂O in minerals at 150° C. Considerable amounts of water are liberated by mineral reactions already in the temperature range 150-250° C also at the above mentioned low geotherms. This behavior determines the rheological characteristics of the upper oceanic crust during early subduction. If water is exclusively released in the sedimentary portion of the downgoing crust only this material gets weakened to be scraped off to form accretionary wedges. At geotherms of 15-20° C/km both lithologies show significant dehydration at very low metamorphic grade. For instance, in cold frontal paleoaccretionary prisms of the Chilean Coastal Cordillera metapelites by far dominate whereas in hotter basal accretionary prisms both low grade basic oceanic crust and continental sediments occur. We also hypothesize that accretionary wedge complexes with a clear dominance of sediments should not have formed in hot subduction zones typical for Precambrian Earth.
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3660 Metamorphic petrology
DE: 8104 Continental margins: convergent
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting